Motorized equipment and vehicles have been specifically designed for many different applications including for use as highway vehicles, farm vehicles and industrial vehicles, such as mobile heavy equipment, which vehicles and equipment utilize fluid power as such is generated onboard using a crankshaft-based internal combustion engine (internal combustion engine) with a rotational hydraulic pump. The main drawbacks of this configuration are its relatively low efficiency and complex design of both the internal combustion engine and the hydraulic pumping system due to the dynamic operating requirements. The flexibility and efficiency of the internal combustion engine could increase significantly if variable compression ratio control can be achieved during the engine operation. Different variable compression ratio mechanisms have been developed, such as based upon variations of piston stroke distances as mechanically constrained by its connection to a crankshaft. However, such developed technologies are subjected to complicated mechanical designs and variable connecting or linking systems. Also, developed variable compression ratio mechanisms suffer from limitations of the response time of an actuation system to cause a variation of piston stroke distance.
An alternative approach is to supply fluid power using a free-piston engine (FPE) with a linear hydraulic pump. Free piston engines offer the ultimate flexibility for variable compression ratio control by eliminating the crankshaft. Such a free piston design also enables advanced combustion techniques such as based upon lower-temperature combustion, which provides better fuel economy and less NOx emissions. Other advantages of a free piston design lies in its simpler design with fewer moving parts, resulting in a compact engine with lower maintenance costs and reduced frictional losses.
Free-piston engine driven hydraulic pumps, for example, can be designed with three different basic architectures: single piston, opposed piston, and opposed chamber arrangement. Single-piston architecture is simple and relatively easy to operate. A single free-piston engine comprises a combustion chamber, a load and a rebound device. With the load being a hydraulic cylinder, the hydraulic cylinder can comprise the load that the rebound device that causes the piston for compression of a successive combustion charge.
Opposed-piston architecture utilizes a common combustion chamber arranged operatively between a pair of single piston devices. Such a design is considered to be self-balanced, and therefore produces no vibration.
An opposed chamber arrangement utilizes a pair of pistons, each associated with its own combustion chamber, which pair of pistons are connected to one another so that one combustion chamber charge moves both pistons in a direction and the other combustion chamber returns both in the reverse direction. Such a design is considered to offer higher power density and therefore a compact design.
A single piston hydraulic free piston engine has been developed within the prior art to reportedly have power output of 17 kW, and indicated efficiencies of nearly 50%. A synchronization method for an opposed piston hydraulic free piston engine design has been proposed according to other prior art systems that combines an electronically controlled hydraulic rebound and a mechanical spring system. According to this method, engine operation is demonstrated with varying power outputs. The efficiency level is shown to be almost constant throughout the power range.
A major technical barrier for bring free piston engines to mass production is the large cycle-to-cycle variation, especially during transient operation. Specifically, the compression ratio of the free piston engine cycle is mainly dependent on the dynamic coupling of the in-cylinder gas dynamics, the load and the piston motion. For a free piston engine design, for example, with 100-mm stoke and 5-mm clearance at the top dead center, a 1% variation of the piston motion (1 mm) will result in a 20% variation in the compression ratio, which will further affect the combustion performance. This imposes a huge challenge on the robust and precise engine operation control. The current free piston engine control methodologies, which are primarily calibration-based, show a limited success and mainly apply to the single piston free piston engine. By calibration-based, it is meant that controls are set for a normal operating mode based upon desired operational conditions and at an effective efficiency. Therefore, systematic active controls and design optimization that can precisely regulate the engine operation are needed.
For conventional internal combustion engines, a crankshaft is the mechanism, which brings the engine back to normal if misfire occurs. Specifically, the crankshaft and flywheel of an engine combine to provide for motion control and energy storage for each piston. Piston motion control creates a desired level of compression. Energy storage provides for the ability to cause a next compression of the next piston.
However, for free piston engines, the combustion and the piston dynamic are heavily dependent on the conditions from last cycle. In other words, a misfire from the previous cycle would result in engine stall in the following cycle. Previous works on free piston engine designs have shown limited success mainly due to the complex dynamic interactions between the combustion and the load in real-time. The systematic stability analysis and control methodology development are not well defined in the prior art.